I. Field of the Invention
The present invention generally relates to the field of polymer-extruder screws for plastic-extrusion machines and in particular to a three-stage-extruder screw.
II. Description of the Prior Art
Plastics are extruded from an extruder bore of a polymer extruder into which polymers in solid pellet and powder forms are fed into a hopper at a proximal end of an extruder bore. In the extruder bore, heat is added and extracted externally as necessary to maintain temperature at an even level while an extruder screw, similar in principle to a meat-grinder screw, mixes, pulverizes, liquefies, pressurizes and conveys the polymer material to a metering gate at a distal end of the extruder bore. It is a three-phase process of (1) mixing, (2) liquefying and (3) metering.
Providing an even flow of plastic in a desired fluid consistency is crucial to efficient and reliable extrusion. Achieving this even flow under dynamic phase-change conditions of the three-phase process has been a major problem in the plastics industry. Improvements are being sought constantly.
Plastic extrusion generally is referred to by originators of some extruder equipment as "plastication". The polymers are said to be "plasticated" by a polymer extruder, although neither of these terms are included in most standard dictionaries. A plastication process differs from plasticization and plasticizing primarily by (1) the use of pulverizing pressure that generates a portion of heat internally for melting the solid pellets, and (2) addition of melt or phase-change heat.
Related improvement steps were described in two U.S. patents, U.S. Pat. Nos. 4,173,417 and 4,356,140 granted to Kruder and U.S. Pat. No. 4,201,481 granted to Iddon et al. The Kruder patents taught a distal-end mixing section of an extruder screw having pairs of two wave channels juxtaposed between helical thread flights of the extruder screw. Wave crests or peaks were offset from wave valleys symmetrically in helical progression.
Crests of the waves did not have constant radii nor near-constant radii in relation to an axis of the extruder screw as one of the features taught by this invention. Instead the Kruder crests were actual peaks having radii which decreased immediately on both helical sides of a maximum height of the crest.
Confirming unawareness of advantages of constant-radii or near-constant-radii crests relative to the axis of the extruder screw, the Kruder patents teach "relatively short" wave cycles. "At least four cycles along the helical length of such channel . . . " is a typical recommendation in the specification and claims of the Kruder patents. Although the option of wave cycles as long as 540 degrees is mentioned in the Kruder technology, there is no reference nor teaching of constant-radii or near-constant-radii crests, regardless of angular duration of wave valleys, whether flat-bottomed or arcuate in helical form.
Further emphasizing crests with radii which change relative to a position separated from the axis of the extruder screw, the second Kruder patent illustrates in its FIG. 6, a wave crest 68 having constant radii in relation to a center of a "protrusion" 62 instead constant radii in relation to a center of the extruder screw 47. Surfaces with changing radii 64 and 66 are shown as having an abruptness that teaches emphatically against graduated inclines of a crest. The inclines 64 and 66, both upstream and downstream helically from the wave crest 68, were circular protrusions, although described contradictorily and incorrectly as inclines. While the second Kruder patent claimed only methods, it was subsequent to the first Kruder patent and demonstrated a tendency not to recognize but to depart further from the channel interruptions and other teachings of this invention.
Although the Kruder description indicated the possibility of different numbers of offset channels side-by-side, its claims limited it to a pair of two. Neither the specifications nor the claims taught the advantages of three with the two outside channels having wave cycles offset from one in the center. To mention a possibility of different pluralities in a description and then to restrict to a particular plurality in claims evidences lack of awareness and, therefore, absence of the teaching of a different plurality, especially when the means for achieving the different plurality require different structure involving constant-radii or near-constant-radii crests as taught by this invention.
The Kruder patents made no mention of equal-depth termination of wave-cycle channels at a distal end of the extruder screw. This is highly advantageous to uniform metering. It is another difference from the Kruder technology.
Although the channels between screw flights in this invention have been referred to previously as channels with "wave-cycle crests and valleys", this is an incorrect and misleading characterization. It is a use of prior-art terms to define new features of the art that require different terms. Instead of wave cycles with crests and valleys in channels, there are channels with "interruptions" by "inclines", "interruption tops" and "core rounds" in helical progression selectively. In some portions of channel interruptions, there is symmetry of incline and decline and in other portions there is not. Also, in some portions of channels, inclines are combined with core rounds to form near-round or near-constant-radii inclines, particularly at top portions of channel interruptions.
Further, yet different, from the Kruder technology, the top portions of channel interruptions have no "secondary flights" between channel crests. Rather, the interruption tops in this invention have no "barrier flights". Barrier flights between channel interruptions are tapered away at channel interruptions to allow passage of materials inwardly towards a center channel or outwardly towards side channels at opposite ends of "interruption cycles". By contrast, the Kruder technology refers to its crests as channels with less depth than at other portions of channels. Its channels continue to be channels having barrier flights as walls in crest sections as in other channel sections of its wave cycles.
The structural differences taught by this invention cause substantial improvement in "plastication" for the following combined reasons. Polymer pellets and powders that have been partially processed in the mixing and transition portions of the extruder screw can be ground, compressed, melted and metered more thoroughly between extruder bores and inclined channel interruptions with this extruder screw than between extruder bores and peaks in shallow channels with the Kruder technology. Less clearance space for more thorough compression and shearing is possible between a channel interruption and an extruder bore than between a channel bottom and an extruder bore. Secondary barrier flights at edges of the channel crests of the Kruder technology prevent effective nearness of crest surfaces to an extruder bore.
Constant-radii channel interruptions taught by this invention provide a fine-grinding effect by selectively long, close relationship to the extruder bore. Near-constant radii of shallow-sloped channel-interruption tops of cycles can be employed to force remaining un-pulverized and un-melted particles more gradually for finer melt results. It is a geared-down rotational grinding effect. Remaining particles are contained over broader surfaces in a smaller clearance space between the channel interruptions and the inside periphery of the extruder bore.
Containment for finer rotational compression is achieved with broader surfaces instead of with secondary barrier-flight walls which necessitate greater space between wave crests and the extruder bore. Particles not trapped and compressed within broader and closer clearance between the interruption tops and the bore are allowed to escape linearly into a center channel or into side channels at alternate cycles for later compression at subsequent interruption tops.
Finally in the metering section, a more thorough, uniform and, consequently, a higher quality of melt without un-melted particles is fed evenly from channels. This is achieved with pressure channels having even depths at a distal end of the extruder screw.
Although employing the principle of an extruder screw as in meat grinders and in the Kruder technology, this is no meat-grinder screw. Nor is it a Kruder screw. Rather, it employs rotational compression much more effectively and for a higher proportion of the work-load of compression than either. A meat-grinder screw has nearly all linear compression which is not effective for plastication of polymer pellets and powders. Linear compression relies on forcing products through restrictions at a distal end of an extruder screw or linearly in reverse between flight tops and a bore in opposition to linear restrictions at the distal end.
By comparison, rotational compression relies on forcing products between a bore and "highs" of crests in the Kruder technology and between a bore and highs of inclines of channel interruptions in this invention. The longer, broader and closer clearances made possible with this invention, together with escape of un-melted particles in either direction for subsequent fine processing, together with other features, combine to provide plastication improvements that are highly significant to the plastics industry.
The Iddon patent taught constant-radii ramps between screw flights. In effect, it employed a plurality of flights having shorter radii between main flights for an extruder screw used in the rubber industry where crushing pellets is not a factor. The flights having smaller radii were bordered by channels extending to a screw core. The radii were constant throughout most of screw length such that there was only minimal if any rotational compression.